The invention relates to a transparent, biaxially oriented polyester film with a base layer B, at least 80% by weight of which is composed of a thermoplastic polyester, and with at least one outer layer A. The invention further relates to the use of the film and to a process for its production.
EP-A-0 878 297 describes a transparent, biaxially oriented polyester film with a base layer B, at least 80% by weight of which is composed of a thermoplastic polyester, and with at least one outer layer A which is composed of a mixture of polymers which contains at least 40% by weight of ethylene 2,6-naphthalate units and up to 40% by weight of ethylene terephthalate units and/or up to 60% by weight of units derived from cycloaliphatic or aromatic diols and/or dicarboxylic acids.
If the outer layer A of the film of EP-A-0 878 297 contains large amounts of ethylene 2,6-naphthalate units, the film has a tendency for delamination between the outer layer A and the base layer B. If, on the other hand, the outer layer A contains low concentrations of ethylene 2,6-naphthalate units, the thickness of this layer has to be raised in order to achieve the low oxygen permeation of not more than 80 cm3/(m2xc2x7barxc2x7d).
In a film in Example 8 of EP-A-0 878 297 the outer layer A uses pure polyethylene 2,6-naphthalate (corresponding to 100% by weight of ethylene 2,6-naphthalate units). In this case there is no significant adhesion between the outer layer A and the base layer B. The film is unsuitable for industrial use (e.g. as a composite film), since the bond releases even when subjected to a low level of mechanical stress, due to the low adhesion between the outer layer A and the base layer B of the polyester film.
In a film in Example 11 of EP-A-0 878 297, the outer layer A contains 60% by weight of ethylene 2,6-naphthalate units. In order to achieve the low oxygen permeation demanded, below 80 cm3/(m2xc2x7barxc2x7d), the thickness of the outer layer A has to be raised to 3 xcexcm, and this is economically disadvantageous (high capital expenditure and high material costs).
U.S. Pat. No. 5,795,528 describes a coextruded film laminate which has alternating layers of PEN and PET. Like the film of EP-A-0 878 297, this film has a tendency toward delamination between the individual layers of PEN and PET. There is no significant adhesion between these layers. A laminate of this type is therefore again unsuitable for industrial use.
It was an object of the present invention, therefore, to provide a transparent, biaxially oriented polyester film which overcomes the disadvantage of the prior art films and in particular has improved adhesion between the individual layers. The film should be simple and cost-effective to produce, have good barrier properties, and pose no problems of disposal.
This object is achieved by means of a transparent, biaxially oriented polyester film with a base layer B, at least 80% by weight of which is composed of a thermoplastic polyester, and with at least one outer layer A, the characterizing features of which are regarded as being that
the outer layer A is composed of a copolymer or of a mixture of homopolymers and copolymers, which contains ethylene 2,6-naphthalate units in an amount in the range of from 90 to 98% by weight and up to 10% by weight of ethylene terephthalate units, and/or units derived from cycloaliphatic or aromatic diols and/or dicarboxylic acids;
the thickness of the outer layer A is more than 0.7 xcexcm and correspondingly makes up less than 25% by weight of the total film; and
the Tg2 value of the polyester film is above the Tg2 value of the base layer but below the Tg2 value of the outer layer.
The film of the invention has low oxygen permeation, below 85 cm3/(m2xc2x7barxc2x7d), and minimum adhesion between the individual layers A and B of greater than or equal to 0.5 N/25 mm.